Randomization by blocks for each participating ICU, using a computer generated list to allocate treatments A or B, put in sealed envelopes near the drug supplies. In each ICU, sealed envelopes including treatment allocation and a five digit number will be available. The envelope will be opened by the person responsible for preparation of the dopamine and norepinephrine solutions. The random number and treatment allocation will be written on a hidden book, available only for the person responsible for preparation of the dopamine and norepinephrine solutions. The doctors and other nurses will be kept blinded to treatment allocation. On patients' charts only the rate of administration and the dilution (simple or double) of the solution will be written.